Conventionally, it is known that an electronic key system controls locking/unlocking of a vehicle door through communicating by wireless between an in-vehicle device mounted in a vehicle and an electronic key unique to the vehicle.
In this electronic key system, the in-vehicle device periodically sends out a signal to the outside of the vehicle, for instance, when a key is not inserted into a key cylinder of the vehicle and furthermore all doors of the vehicle are locked. When a driver having an electronic key is near the vehicle, a response signal to the signal sent from the in-vehicle device is returned from the electronic key. As the in-vehicle device receives the response signal, it executes authentication with the electronic key. When the in-vehicle device successfully completes the authentication and thereafter detects that a hand is put into a doorknob, it automatically releases locking of the doors.
Incidentally, an antenna of the in-vehicle device or the electronic key is typically formed of an antenna coil and an external capacitor. The antenna coil is formed by winding electric wire around a stick ferrite core. The external capacitor constitutes a parallel resonance circuit with the antenna coil. However, when the reception antenna of the electronic key is formed of a single antenna coil, a communication distance (where data from the vehicle can be received) extremely decreases depending on relationship with a direction of a magnetic field generated by a transmission antenna of the in-vehicle device. At worst, the communication becomes impossible.
In detail, the reception antenna of the electronic key is most sensitive when an axial direction of the antenna coil of the reception antenna is parallel with the direction of the magnetic field generated by the transmission antenna of the in-vehicle device. That is, an electric voltage is most efficiently induced in the antenna coil of the electronic key. By contrast, the reception antenna of the electronic key is least sensitive when the axial direction is orthogonal to the direction of the magnetic field. That is, the electric voltage is hardly induced in the antenna coil of the electronic key.
The reception antenna of the electronic key needs to stably receive the signal from the in-vehicle device irrespective of the relationship with the direction of the transmission antenna of the in-vehicle device. The reception antenna needs to be formed into being nondirectional using a plurality of antenna coils.
Constructing of the reception antenna with the plurality of the antenna coils involves a large volume for disposing the reception antenna. Closely disposing the plurality of the antenna coils in the limited volume inside the electronic key may result in lowering communication performance due to mutual interference among the antenna coils.